The objectives of this research program are to determine the processes which are important in the induction and progression of murine mammary "prenoplastic" or high-risk lesions. Several model systems, both in vivo and in vitro, have been developed and are being analyzed to pursue these objectives. The use of chemical carcinogens to induce transplantable, ovarian dependent, ductal hyperplasias and neoplasias provides a system which has similarities to human breast cancer and an improved system to examine the role of estrogens in the control of neoplastic growth. The use of monolayer cell cultures of normal, preneoplastic, and primary mammary neoplasms provides a system to examine cell surface, cellular and biochemical characteristics of the neoplastic state, and avoid problems of interpretation due to cellular changes due to neoplastic progression. The in vitro systems hopefully will provide information on the types of cells which comprise preneoplastic cell populations and the effects of agents which promote or inhibit neoplastic transformation.